Tens of thousands of supermarket workers are on strike in southern California, seeking higher wages and improved benefits from their employers.

Many markets have hired replacement workers to keep stores open despite the walkout.

Negotiations between the workers' union and management broke down during the weekend, and the strike began late Sunday.

One of the big issues keeping the two sides apart is health insurance. Managers of the big supermarket chains - Vons, Ralphs and Albertsons - say workers will have to start paying a share of health insurance costs, which until now had been borne completely by employers.

Union workers walked off their jobs late Sunday at one group of stores, and managers locked them out at another.

Up to 70,000 union workers and 850 grocery stores are affected by the dispute.